The present invention relates to capacitive touch switch arrangements and more particularly to a novel touch switch arrangement employing only one electrode on the opposing surface of a dielectric substrate opposite each touch electrode to accommodate a greater number of touch electrodes in a given surface area.
Capacitive touch control panels are a well known means for providing user control inputs to various devices, including major home appliances such as cooking ranges and microwave ovens, which have capacitive touch switches arranged in a keyboard array. With such touch panels, the user merely touches a touch area on the panel to initiate a switching action rather than moving a mechanical switch.
Capacitive touch systems can generally be classified in two types. In one, the switch cells each comprise a single capacitor formed of two conductive plates located on a single surface of a dielectric material. The capacitor is energized from a pulsating signal source and is coupled to a signal detection circuit, the detected signal being a function of the capacitance which is changed in response to touch. The switch cell of the other type of capacitive touch system each comprises one pair of series connected capacitors formed of three conductive plates, two closely spaced on one surface of a dielectric material and the third plate on the opposing surface overlapping the other two plates. In this device, the series connected capacitors, which are energized by an AC signal source, form a voltage divider arrangement with the stray capacitance at the input to the signal detector. Touching the third plate alters the capacitive voltage divider arrangement, thereby changing the voltage level at the detection circuit as a function of touch.
This latter type of system is capable of good signal characteristics and an effective and reliable operation when sufficient signal is capacitively coupled through the series capacitor pair of each device. Satisfactory operation generally requires some minimum value of capacitance for the series capacitors, as well as relatively small parasitic capacitive effects. Since, for a given dielectric material, the series capacitance is proportional to the capacitor plate area, presently known switch devices of the capacitance attenuator type for practical appliance control applications have minimum area requirements that tend to preclude their application to switch arrays needing a relatively large number of switch cells within the constraint of a relatively small panel surface area.
A touch panel arrangement which provides the performance capability and employs the well developed interfacing circuitry of the three-plate type of capacitive touch system but which is applicable to switch array applications requiring a relatively large number of switch cells in a relatively small panel area would be highly desirable.